1. Field of the Invention
The present invention relates to a control apparatus for a thyristor motor and more particularly to a control apparatus for a thyristor motor having a compensation field coil for generating compensation field flux orthogonally intersecting with main field flux so as to compensate armature reaction.
2. Description of the Prior Art
A thyristor motor is a synchronous motor driven by semiconductor commutator means. FIG. 1 is a block diagram showing a conventional control apparatus for a thyristor motor. Although the below described direct current I.sub.d, DC voltage E.sub.d, main field current I.sub.f, compensation field current I.sub.c etc. in a real state and signals for detecting or instructing them do not have the same values, the real values and the signals will be sometimes regarded as the same in the present specification for the purpose of facilitating the explanation. The synchronous motor 3 has armature coils U, V and W and field coils 310 comprised of a main field coil F and a compensation field coil C. The main field coil F generates main field flux and the compensation filed coil C generates compensation field flux orthogonally intersecting with the main field flux. To the rotating axis of the synchronous motor 3, a position sensor 4 and a tachometer generator 6 are connected. The position sensor 4 provides a position signal of a phase according to the rotational angle of the rotating axis of the synchronous motor 3. The tachometer generator 6 generates voltage proportional to the rotational speed of the rotating axis of the synchronous motor 3. A control apparatus for a thyristor motor comprises in rough a power supply circuit 100, an inverter circuit 200, an excitation circuit 70 and a speed instructing circuit 900. The power supply circuit 100 comprises a converter 1, a current detector 9, a current controller 10, a gate pulse phase shifter 11 and a coefficient multiplier 20. The converter 1 converts AC power of a commercial AC power source into DC power. The current detector 9 rectifies AC input current of the converter 1 and provides a signal proportional to the DC current I.sub.d outputted from the converter 1. The coefficient multiplier 20 multiplies, by a predetermined coefficient, an instructed value of torque outputted from a speed controller 8 to be described below so as to provide an instructed value of current for the converter 1. The current controller 10 amplifies a deviation between an output signal of the coefficient multiplier 20 and an output signal of the current detector 9. The gate pulse phase shifter 11 controls an ignition phase of the converter 1 according to the output signal of the current controller 10. The inverter circuit 200 comprises an inverter 2 and a gate amplifier 5. The gate amplifier 5 provides a gate signal to the inverter 2 based on a position signal from the position sensor 4. The inverter 2 commutates the DC power from the power supply circuit 100 in response to the gate signal so as to supply the power to the armature coils U, V and W. The speed instructing circuit 900 comprises a speed instructing device 7 and a speed controller 8. The speed instructing device 7 provides a speed instructing signal for making the rotational speed of the synchronous motor 3 be a predetermined rotational speed. The speed controller 8 examines and amplifies a difference between the speed instructing signal from the speed instructing device 7 and a speed feedback signal from the tachometer generator 6. The excitation circuit 70 comprises a main excitation circuit 71 and a compensation excitation circuit 72. The main excitation circuit 71 comprises a field instructing device 12, a coefficient multiplier 21, an adder 22, a current detector 13, a current controller 14, a gate pulse phase shifter 15 and a converter 16. The field instructing device 12 instructs a no-load value I.sub.fo of the main field current I.sub.f. The coefficient multiplier 21 multiplies by a predetermined coefficient, an instructed value of torque outputted from the speed controller 8 so as to apply a correction amount .DELTA.I.sub.f for a demagnetized amount of the main field current I.sub.f in the loaded condition. The adder 22 performs addition of an instructed value of field current outputted from the field instructing device 12 and a correction amount outputted from the coefficient multiplier 21 so as to obtain an instructed value I.sub.fp of field current represented by the equation I.sub.fp =I.sub.fo +.DELTA.I.sub.f. The current detector 13 rectifies the AC input of the converter 16 for control of the main field so as to detect the amount of main field current I.sub.f. The current controller 14 amplifies a deviation between the signal I.sub.fp and a detected value of current outputted from the current detector 13. The gate pulse phase shifter 15 controls an ignition phase of the thyristor in the converter 16 according to the output of the current controller 14. The converter 16 supplies a main field current I.sub.f in response to a signal from the gate pulse phase shifter 15. The compensation excitation circuit 72 comprises a coefficient multiplier 23, a current detector 17, a current controller 18, a gate pulse phase shifter 19 and a converter 24. The coefficient multiplier 23 multiplies an instructed value of torque outputted from the speed controller by a predetermined coefficient so as to provide instruction of current for compensation field. The current detector 17 rectifies the AC input to the converter 24 for control of compensation field so as to detect the amount of compensation field current I.sub.c. The current controller 18 examines and amplifies a difference between an instructed value of compensation filed current outputted from the coefficient multiplier 23 and a detected value of current outputted from the current detector 17. The gate pulse phase shifter 19 supplies ignition pulses to the thyristors in the converter 24 according to the output of the current controller 18. The converter 24 supplies a compensation field current I.sub.c in response to a signal from the gate pulse phase shifter 19.
Now, description will be made of a total operation. The position sensor 4, the gate amplifier 5 and the inverter 2 operate so that the phase of the armature current I.sub.a of the synchronous motor 3 may be a predetermined phase with respect to the rotating phase of the field flux. The tachometer generator 6, the speed instructing device 7 and the speed controller 8 provide instruction of torque so that the rotational speed of the synchronous motor 3 may be equal to the instructed speed. The coefficient multiplier 20 multiplies the instruction of torque by a coefficient determined by various constants of the synchronous motor 3 so as to instruct armature current I.sub.a necessary for generating torque equal to the instructed value. The process in which the direct current I.sub.d is controlled to a predetermine value by the current detector 9, the current controller 10, the gate pulse phase shifter 11 and the converter 1 is well known. The field instructing device 12 supplies a reference value I.sub.fo of the main field current in the no-load condition, and this reference value becomes an instructe-d value of current I.sub.fp after a field current increment .DELTA.I.sub.f for correction of a demagnetized amount in the loaded condition is added to the reference value. The process in which the main field current I.sub.f is controlled to be a predetermined value by means of the current detector 13, the current controller 14, the gate pulse phase shifter 15 and the converter 16 is well known. The coefficient multiplier 23 instructs compensation field current I.sub.c necessary for compensation of the armature reaction determined by the constants of the motor. The instructed torque and the armature current I.sub.a are maintained in a proportional relation and the compensation field current I.sub.c and the instructed torque are also maintained in a proportional relation. Accordingly, the armature current I.sub.a and the compensation field current I.sub.c are controlled in proportion to each other. The current detector 17, the current controller 18, the gate pulse phase shifter 19 and the converter 24 control the compensation field current I.sub.c according to the instructed value.
FIG. 2A is a vector diagram showing a relation between the voltage and the current of the FIG. 1 motor in the no-load condition. FIG. 2B is a vector diagram showing a relation between the voltage and the current of the FIG. 1 motor in the loaded condition. The inverter 2 is an external commutated inverter and, therefore, it is necessary to provide current in a leading power factor for commutation. For this reason, the position sensor 4 is disposed in the synchronous motor 3 so that the armature current I.sub.a may flow in the direction advancing by an angle .gamma. with respect to the no-load induced voltage E.sub.o. In the loaded condition, the armature current I.sub.a causes voltage X.sub.s I.sub.a in the direction shown in FIG. 2B due to the armature reaction. The voltage X.sub.s I.sub.a includes a direct-axis component and a quadrature-axis component. Voltage X.sub.s I.sub.c caused by the compensation field coil is generated in the direction shown in FIG. 2B. This voltage X.sub.s I.sub.c compensates the quadrature component of the armature reaction. I.sub.f this state continues, the induced voltage V in the loaded condition becomes smaller than the induced voltage E.sub.o in the no-load condition and as a result, a sufficient output of the motor cannot be obtained. For this reason, voltage X.sub.s .DELTA.I.sub.f is generated by increasing the main field current by an amount .DELTA.I.sub.f, whereby the induced voltage having the same amount as that in the no-load condition can be obtained.
As is understood from the foregoing description, in a conventional control apparatus, a compensation field current I.sub.c and a correction value .DELTA.I.sub.f of the main field current are made to flow in proportion to the armature current I.sub.a so that the armature reaction may be compensated. In such a method, a precise compensation can be made as far as a vector relation as shown in FIG. 2B is maintained, and since the torque generated in the motor is proportional to the armature current I.sub.a, torque control can also be made with precision. However, in reality, it is well known that commutation of the inverter 2 is not provided instantaneously and that an overlapping angle of commutation is caused. As a result, a delay from the determined angle .gamma. is caused in the phase of the armature current I.sub.a and this phase delay becomes a significant amount as the frequency (the rotational speed of the motor) becomes high. Furthermore, the larger is the armature current I.sub.a, the greater is the phase delay and accordingly, deviation in the vector relation changes according to the change in the instruction of torque.
Thus, the phase .gamma. of the armature current changes according to the changes in the rotational speed of the motor or in the armature current and, therefore, the direction of the armature reaction changes. Accordingly, the armature reaction cannot be compensated with precision by applying the compensation field current I.sub.c and the correction value .DELTA.I.sub.f of the main field current. As a result, deviation is caused both in the amount and in the phase of the induced voltage V in the loaded condition with respect to the no-load induced voltage E.sub.o. As described above, a conventional control apparatus has a disadvantage in that a phase relation between the armature current I.sub.a and the induced voltage V cannot be maintained in a loaded condition and accordingly torque cannot be obtained in accordance with the instruction of torque.